Ask Mr. Science
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Will Pluto and Neptune ever collide?

Great question! In all these years of frequently talking about the planets, this question had never come up.

The orbit of Pluto is highly elliptical, and Pluto is sometimes closer to the Sun than Neptune. So when the orbits cross, will they ever collide?

There are several reasons why the answer is 'NO'. One is that Neptune and pluto are in '3:2 resonance orbits', that is, neptune goes around the sun 3 times in the same time that pluto goes around 2 times. This prevents Pluto from coming arbitrarily close to Neptune.

But the best explanation in my opinion I found on the 3rd link below (the one from NASA). Not only is the orbit of Pluto not very circular, it is also tilted a lot relative to the plane in which Neptune, and all the other planets, orbit. If you look at the solar system side-on, and make a plot of every planet's height above the plane versus distance from the Sun, you see that when Pluto crosses Neptune's orbit, it is high above the plane, and when Pluto crosses the plane, it is far beyond Neptune.

October 2007

Why do we have leap years?

I backed up a bit and asked

  • What's a day? When does it start? How long is it?
  • What's a week? How many days? Why?
  • What's a month?    Is it till fixed to the Moon? How long? How many in a year?
  • What's a year? How many days? How many months? When does it start?

    For each of these, there are times and places where the duration, starting point, and number vary, and if you follow the links below you can find all the answers.

    Days: there are different days: the modern midnight to midnight in your timezone, or in the past local noon to noon, sunset to sunset, sunset to sunrise, but all of these correspond to one rotation of the earth relative to the Sun. In the links you'll find stories about how local times varied from city to city before the age of trains. How in Jewish ans islamic traditions, days start at sunset. The modern definition of a day is 86400 seconds, or 86399 if a leap second is subtracted, or 86401 if a leap second is added.

    Weeks: Weeks are mostly arbitrary, usually the interval between market days, or between (religious) days of rest. There have have been 'weeks' of 5, 6, 7, 8, 10 days, most of the time not tied to months or years. Some 'weeks' were tied to the moon:7 days from new moon to first quarter, second 7 days from first quarter to full moon, etc, plus at the end of the month a one or two leap days that don't belong to any week, until the next new moon starts the next set of 4 weeks.

    Months: As the name suggests in all languages, months are connected to the lunar cycles.In some calendars such as the islamic calendars, they are still strictly tied to observed or calculated cycles of the moon. If you define a year as 12 moons, as in the islamic calendar, and since 12 moons are less than 365 days, the islamic new year shifts around the solar year. Sometimes newyear is in the summer, sometimes in the spring, winter or fall. This is a strictly lunar calendar. If you want to keep some connection between lunar months and the solar year, you can throw in a leap month now and then, as in the jewish calendar, such that for example the jewish passover holiday always falls near the spring equinox. This mixed system is call a lunisolar calendar.

    Year: In most cultures, the year is a cycle of days meant to keep in step with the seasons. The seasons are determined by the path of the earth around the sun. This is important if you need to know when it is time to plant, when the fish and birds are returning. A problem arises since a solar year is not an exact number of days. It it (currently) 365.2421896698 days. Julius Caesar's scientist had determined that is was 365 1/4 days, pretty good for their time. To take care of the 1/4 day, they instituted the leapyear every 4 years. After some fumbling this, system was used for centuries. However, the small discrepancy between 365.242... and 365.25 kept on adding up, and by 1582 Pope Gregory XIII decreed a one-time correction of 11 days, plus a system where leap years are skipped every 100 years (such as 1800, 1900 which were not leap years), with the exception of years divisible by 400 (such as 2000, which was a leap year).
    Besides the length of the year, you have to choose where the year starts. The names of some months (September-December) show that the year ised to start on March 1. Other days were December 25, and Easter. It is January 1 on the modern Western calendar.

    Seconds: The second originally was 1/60th of a minute, which was 1/60th of an hour, which in turn was 1/24th of a day. The current definition says that one second equals 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. The actual length of a day varies a little due to unpredictable variations in the rotations of th Earth, so occasionally a leap second has to be added or subtracted to keep our clocks synchronized with the Earth's rotation.

    Dec 2007

    Fun with liquid nitrogen

    I had recently acquired a 4-liter dewar bottle (from the famous 'Black Hole' in Los Alamos), so it was time to play with liquid nitrogen (LN2). My local gas company will fill up my bottle for about $20, and it keeps for days, so it is not too extravagant, and you can get some on Friday and have plenty on Sunday to prepare the experiments. I did some of the following things in class:
    1. Put some LN2 in a small styrofoam cooler. Blow up a balloons and put them in - put the lid back on. Blow up another and add it. Since the LN2 will shrink the balloons, you can fit a whole bunch in. Perhaps the best way is to blow them up first, so you can see how much more volume ten balloons are relative to the cooler into which they will disappear. As the hour progresses, they will slowly self-inflate and one by one bulge out of the cooler.

      With your glove, take out a balloon that is fully shriveled. Almost immediately, it starts re-inflating itself. While that is going on, hold it up high and notice a little bit of liquid inside the balloon. What is that? It's liquid oxygen (see below)! Maybe show this after you've done the liquid oxygen demo. See if they recognize what's going on.

    2. Pour some on a serving tray. You can see the liquid scooting around the surface, boiling as it goes. The drops scoot around virtually frictionlessly, as they ride on a layer of vapor where the drops touch the (relatively) hot surface. Same effect as water drops in a hot dry skillet. See a movie.

    3. While you have LN2 in the cooler, put a banana in it. When it is good and cold, you can use the banana to hammer a nail into a piece of wood . Choose soft wood, and a nail with a big head to ensure this goes smoothly. [I noticed that the first time I froze the banana and did it, it stayed whole. Then I kept it in the freezer for a few days, and on demo day I used it again; this time it broke into pieces, still big enough to do the job though.] Make sure you wear a glove when you do this one, and keep an eye on the frozen pieces - they stay cold for hours, and can do damage to unprotected skin. Don't leave the banana behind in the classroom. When it warms up, it will turn to mush, so it is only good for making banana bread.

    4. Pop a lid off a can I had a small cup with a tight-fitting lid. Pour some LN2 into the cup and jam the lid on quickly. The boiling LN2 will pop the lid off. You can do this several times before the liquid is gone. Careful with the cup - it is too cold to handle. And find a cup that blow the lid straight up every time, so no one get hurt.

    5. Shatter something I didn't actually do this, but a flower or a small rubbber ball can be crushed after cooling. Rubber bands don't shatter. A tennisball shatters if you stomp on it, but all the pieces are held together by the outside fibers. Then you have to cut it open, which is not so flashy.

    6. Make liquid oxygen! Oxygen liquifies at a higher temperature than nitrogen. So just like water condenses on the outside of a glass of ice water, oxygen condenses on the outside of a liquid nitrogen vessel. On the photo at the top of this section you can see a simple setup that works nicely: Take a piece of aluminum foil and fold it to make a pleated cone. Tape this to the legs of your tripod, high enough for a cup to fit under. When you fill the cone with LN2, liquid oxygen (LO2) will drip off the outside into the cup. [Note in the picture I use small dewars, but styrofoam cups (several nested) should work too]. You can collect a cc in a few minutes.

      How can you tell this is liquid oxygen?

        A  LNO2 is a magnetic liquid! I took a small strong magnet and stuck it on the end of a steel rod. I precooled it in some LN2. It comes out clean. However, when you then dip it into the oxygen, a blob of liquid will hang onto the magnet, as shown in the photo.
        B  LO2 accelerates combustion. In the photos on the right, I have a glowing woodchip, which burst into bright flame when you touch it to the liquid oxygen.
    7. When I was young, I saw a demo of a lead bell [in the Evoluon in Eindhoven, anybody remember that one?] Of course a warm lead bell doesn't sound at all, but when cooled, it really rings nicely.

    8. Miniature marshmellows. Soak them in LN2, fish them out with the tongs, and hand them out. They now are crunchy, and if you pop them quickly into your mouth, you can make smoke come out of your nose. How cool is that in 6th grade! Their heat capacity is so low they cannot hurt anything.

    9. Not yet tried: (a) Make a vapor-filled soap bubble. Need a bowl, a separate cup with soapy water and a strip of cloth. (b) Apparently, an icecube cooled to LN2 temperatures has shrunk enough to sink. Should be easy to do, since I already have icecubes with me to show water phases and condensation.

    Some basics

    Temperature is a measure of how energetic and agitated atoms or molecules are. In a gas, the colder it gets, the slower the atoms fly around. Thus you can imagine a temperature where they would come to a standstill. This defines the bottom of the temperature scale. In reality, you can't quite get to absolute zero, but you can get arbitrarily close. There is no maximum to the temperature scale, since you can always add more energy to any system, and make atoms fly faster.

    We can run across three different temperature scales: Kelvin (°K), Celsius (not centigrade) (°C)),and Farenheit (°F).

    • The Kelvin scale is the one used by scientists, because 0°K is at absolute zero. The size of the scale is the same as the size of the Celsius scale: the difference between boiling and freezing water is 100 degrees on both scales. The scales are just shifted relative to each other by 273°.
    • The Celsius scale has the zero at the freezing point of water, and the boiling point of water is defined as 100°C. Absolute zero is at -273°C.
    • The Farenheit scale is used only in a few countries. Mr. Fahrenheit put the 0 and 100 at the lowest and highest temperature in the natural world: 0F is where a mix of water, ice and salt is in equilibrium, and 96 was the body temperature of a person. Why not at 100? Read the long story at Wiki. For scientists, the persistence of this scale is an irritating embarassment, as are inches and miles.
    What to bring:
    • Dewar with LN2
    • A (balsa) stick to measure the LN2 level
    • Gloves
    • Goggles
    • Styrofoam cups.
    • A glass and
    • Thermos with water and ice cubes. With this, you can show different phases (of water), and show condensation on a cold surface.
    • Balloons
    • Banana
    • Tripod
    • Aluminum foil folded into a pleated cone
    • Masking tape
    • Wood chips,
    • Candle
    • Lighter or matches.
    • Small strong magnet
    • Metal rod (coathanger)
    • Wood block
    • Nail
    • Serving tray
    • Flower to crush
    • Miniature marshmellows and
    • Tongs for grabbing them.

    January 2008, October 2008, October 2010

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    Last update 15 May 2005 - HvH
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